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Re: LIKE query on indexes

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
Cc: Ibrahim Tekin <itekin(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: LIKE query on indexes
Date: 2006-02-21 16:34:16
Message-ID: 20060221163416.GC6541@surnet.cl (view raw or flat)
Thread:
Lists: pgsql-performance
Scott Marlowe wrote:
> On Tue, 2006-02-21 at 09:57, Ibrahim Tekin wrote:
> > hi,
> > i have btree index on a text type field. i want see rows which starts
> > with certain characters on that field. so i write a query like this:
> > 
> > SELECT * FROM mytable WHERE myfield LIKE 'john%'
> > 
> > since this condition is from start of the field, query planner should
> > use index to find such elements but explain command shows me it will
> > do a sequential scan. 
> > 
> > is this lack of a feature or i am wrong somewhere?
> 
> This is an artifact of how PostgreSQL handles locales other than ASCII.
> 
> If you want such a query to use an index, you need to back up your
> database, and re-initdb with --locale=C as an argument.

... or you can choose to create an index with the text_pattern_ops
operator class, which would be used in a LIKE constraint regardless of
locale.

http://www.postgresql.org/docs/8.1/static/indexes-opclass.html

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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